With a shell account on an arbitrary POSIX semi-compliant system, one should
have access to a Bourne-like Shell, awk, dc,
sed and companions. Given a source of randomness this should
be sufficient to code RSA + a symmetric cipher, kind of extremely poor man's
PGP.

I had some problems finding ways to output binary stuff from ksh.
UPDATE: New version seems to work with bash.

On Intel at 1.6 Ghz it encrypts/decrypts at 184 Bytes per second.
One optimization could be to put the keystream generation entirely in a dc script,
start that in a sub-process, and read single bytes from a fifo.
UPDATE: New version does this, 370 Bytes/sec now.